Keywords:

Pohlsepia mazonensis gen. et sp. nov. from the Mazon Creek Konservat Lagersta¨tte (Carboniferous) of Illinois is an exceptionally preserved soft-bodied fossil coleoid, with well-defined body and arms. Lacking an internal shell and possessing eight subequal and two modified arms, Pohlsepia can be compared with both the living cirrate octopods and the decabrachian sepiardarids, both of which lack a well-developed internal skeleton. Given its sac-like body, lack of a well-defined head and presence of fins, Pohlsepia can be safely compared with modern cirrate octopods. It is the oldest known completely soft-bodied coleoid and as such has great significance with respect to the phylogeny of the group, given that both the octobrachian and decabrachian clades have previously been thought to have evolved in the Jurassic. Key words: Coleoidea, Octobrachia, Konservat Lagersta¨tte, Mazon Creek, Carboniferous.